The present invention is a magnetic displacement sensor having a magnetic circuit reluctance that is constant with displacement. More specifically, the present invention is a magnetic displacement sensor having improved flux shaping pole pieces for improved performance.
Typically, magnetic displacement sensors include a flux generator that provides a constant source of magnetic flux and a pickup device that measures flux. Typically, the flux generator is mounted to one element and the pickup device is mounted to another element so that the magnetic flux density sensed by the pickup device is based on the displacement between the elements. Magnetic displacement sensors typically measure linear or rotational displacement and provide an output proportional to absolute linear or rotary position displacement of the elements. Magnetic displacement sensors may employ either electromagnets or permanent magnets as a source of magnetic flux. A pickup device (e.g., a magnetoresistor, a magnetodiode, or a Hall effect sensor) intersects the magnetic flux and detects changes in the magnetic field produced by the magnets.
Magnetic displacement sensors are commonly used in cooperation with microprocessors in remote control systems with field devices. For example, magnetic displacement sensors can be used to monitor valve position. Examples of prior art magnetic displacement sensors are found in Prinz et al. U.S. Pat. No. 4,532,810, Wolf et al. U.S. Pat. No. 5,497,081, and Riggs et al. U.S. Pat. No. 5,359,288.
The useful range of prior art magnetic displacement sensors is limited by the magnets"" fringing flux. When two permanent magnets are adjacently aligned, the mechanical characteristic of each magnet and its proximity to the other magnet pole face dictate the distribution of magnetic flux from pole face to opposite pole face. The fringing flux appears between the magnetic poles of the two-magnet assembly. Where a single magnet is used as the flux generator, fringing flux extends between the opposite poles of the magnet are is concentrated quite close to and parallel to the magnet. In both cases fringing flux variations along the length of the magnet(s) are non-linear. Therefore, the magnetic field detected by the magnetic displacement sensor varies non-linearly with displacement. This non-linearity results in inaccurate and erroneous sensor readings, limiting the useful range and effectiveness of magnetic displacement sensors.
The two magnet assembly is also susceptible to errors due to relative rotation of the magnet assembly and the sensor. It is typical, for example, to mount the magnet assembly to a moving part whose lineal position is being monitored, and to mount the sensor to stationary housing for connection to control circuitry. If the moving part rotates during control or positioning operations, the magnet assembly might rotate to a position where the magnetic field of the magnet assembly no longer correctly operates the sensor.
The present invention is directed to flux-shaping pole pieces for the magnet pole faces of a magnetic displacement sensor.
One form of the invention is a displacement sensor that senses relative displacement between first and second elements. A magnet assembly is mounted to the first element and defines a longitudinal space having an axis along its length. The magnet assembly includes at least one magnet having a north pole and a south pole that supply fringing magnetic flux in the longitudinal space and a flux-shaping pole piece on each of the north and south poles. The flux-shaping pole pieces have configurations to sculpt fringing magnetic flux in the longitudinal space so that magnetic flux density in the longitudinal space varies substantially linearly along the axis of the space. A magnetic field sensor assembly has a sensor housing for mounting to the second element and a magnetic flux sensor mounted to the sensor housing on the axis of the longitudinal space. In a preferred form of the invention each of the flux-shaping pole pieces has a pentagon shape in a plane of a primary flux pattern in the longitudinal space, the pentagon shape forming a first face confronting the longitudinal space parallel to the axis of the longitudinal space.
In one embodiment of the invention, the magnet is a cylindrical magnet and the longitudinal space is adjacent to and extends along the length of the cylindrical magnet. In this form of the invention, the first face of each flux-shaping pole piece is cylindrical, and the pole piece is oriented so that the cylindrical first face is parallel to and coaxial with the cylindrical magnet. Preferably, the pole piece has a frusto-conical face between the cylindrical face and the second face and confronting the longitudinal space.
In one use of the invention, one of the first and second elements is a valve actuator and valve stem housing and the other of the first and second elements is a stationary housing so that the displacement sensor senses linear displacement between the valve stem and the stationary housing.
Another form of the invention is a flux-shaping pole piece for a pole of a magnet of a magnetic displacement sensor for sensing relative displacement between first and second elements, wherein the magnet forms a longitudinal space substantially parallel to a length of the magnet. The pole piece comprises a magnetic material having an attachment surface for attachment to the pole of the magnet. The pole piece is configured so that when the pole piece is attached to the poles of the magnet, each of the pole pieces sculpt fringing magnetic flux in the longitudinal space so that magnetic flux density in the longitudinal space varies substantially linearly along the axis of the longitudinal space.
In a preferred form of this embodiment of the invention, the magnet is a cylindrical magnet having an axis, and the pole piece has a pentagon shape in a plane of primary flux pattern across the longitudinal space, the pole piece has a cylindrical face confronting the longitudinal space and coaxial to the axis of the magnet when the pole piece is attached to the magnet pole. Preferably, the flux-shaping pole piece includes a frusto-conical face confronting the longitudinal space between the cylindrical face and the attachment surface.